A secondary battery is a battery which is able to provide electricity by converting chemical energy into electrical energy; moreover, it is a battery which is able to store (during charge) chemical energy by converting electrical energy into chemical energy by passing an electric current in a direction that is opposite to the discharge direction. Among secondary batteries, lithium ion batteries have higher energy density when compared to other chemistries such as Lead-Acid, Nickel-Cadmium and Nickel-Metal Hydride, and are thus widely used as a power source for notebook personal computers, cellular phones and other portable devices.
In a lithium secondary battery using graphite (C) as the negative electrode active material, the reaction described by the following formula (I) proceeds at the negative electrode at the time of discharge:LixC→C+xLi++xe−  (I)In this formula, 0<x<1.
An electron produced by the formula (I) passes through an external circuit, which acts as an external load, and then reaches the positive electrode. At the same time, a lithium ion (Li+) produced by the formula (I) is transferred through the electrolyte sandwiched between the negative and positive electrodes from the negative electrode side to the positive electrode side.
When lithium cobaltate (Li1-xCoO2) is used as a positive electrode active material, a reaction described by the following formula (II) proceeds at the positive electrode upon discharge:Li1-xCoO2+xLi++xe−→LiCoO2  (II)In this formula, 0<x<1.
Upon charging the battery, reactions which are reverse to the reactions represented by the above formulae (I) and (II) proceed at the negative and positive electrodes. The graphite material in which lithium was intercalated (LixC) becomes reusable at the negative electrode, while lithium cobaltate (Li1-xCoO2) is regenerated at the positive electrode. Because of this, discharge becomes possible again.
Conventional lithium secondary batteries use a flammable, volatile organic compound as an electrolyte solvent; therefore their safety is limited.
As an effort to increase the safety of electrolytic solutions, lithium secondary batteries using an ionic liquid as an electrolyte are conventionally known. An “ionic liquid” is a salt that is liquid at 100° C. or less, and it is generally nonflammable and nonvolatile. Such a nonflammable electrolyte has several advantages, in that it is able to not only increase the safety of the battery but also has a relatively wide potential window (stability range) and shows relatively high ion conductivity.
As an ionic liquid technique, Patent Literature 1 discloses chiral ionic liquids having, as the anion, an anion of an organic or inorganic proton acid and, as the cation, an optically active organic ammonium cation with at least one chirality center and at least one functional group, the chirality center being provided with a distance of up to 5 atomic bonds from the functional group and the functional group being selected from alcohol and so on and able to produce a coordination by forming hydrogen bridges or by providing free electron pairs.